Fix for double backwards tests (#18190)
Summary:
If none of the outputs require_grad, we don't actually check gradgrad, instead we will check that their numerical gradients are 0.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18190
Differential Revision: D14563388
Pulled By: ifedan
fbshipit-source-id: a4eb94c9eb60f14dbe6986cd8cef1fe78a7bc839
